Inks used for gravure printing are comprised of a colorant, a binder and a solvent. It is crucial to the performance of gravure inks that they have the correct flow characteristics, in particular the correct viscosity. This is important in the inking of the recessed cells of the etched or engraved printing cylinder and the delivery of the ink from the cells of the plate to the substrate. The viscosity of the ink is also important in order to achieve an acceptable degree of holdout (resistance to penetration) of the ink when printed on paper, especially uncoated paper stock having high porosity. The lower the ink viscosity the more severe is the problem of lack of holdout.
The proper ink viscosity can be easily achieved by the use of greater amounts of binder and lesser amounts of solvent, but this increases the overall cost of the final ink. Also, use of large amounts of binder to obtain the desired viscosity means that in the final thinning of the ink by the printer less solvent can be employed, giving the printer less latitude in his formulations. The inks which cannot readily be diluted are also perceived by printers to have "low mileage," that is, less paper coverage per gallon. Printers prefer ink that can be diluted with greater amounts of solvent because of the benefits of economy of the final ink formulation and convenience in the formulation process.
The term "dilution" is a term of art used by ink formulators to describe the amount of solvent required to thin a given ink composition to a desired viscosity. The term may also be used for unpigmented resin solutions generally referred to as varnishes. In this context, the dilution of a resin or varnish is related to the property of "intrinsic viscosity" as used in the polymer art, that is, the higher the resin molecular weight, the higher the viscosity of solvent solution at lower concentrations and, therefore, the higher its possible dilution.
Metal rosin resinates have commonly been employed as ink binders in the formulation of gravure inks. The resinate serves to provide the ink with the necessary viscosity, transfer, printed gloss and rub resistance. However, achieving the desired high dilution with a metal rosin resinate alone has been difficult if not impossible to achieve because of the generally very low molecular weights typical of this class of resins.
In particular, desirable high dilution values in the range of 90-100 mls toluene, to reach print viscosity of about 7.5 cps as measured from 50% solids concentration, can be achieved only by neutralizing the resinate system to nearly 100% of theoretical with zinc oxide, magnesium oxide, and/or calcium hydroxide. This, however, results in unacceptably high resinate viscosity and severe viscosity instability. In other words, the high dilution resinates can be made using conventional resinate formulations but they are too viscous to use conveniently, are difficult to manufacture, and are prone to increase further in viscosity during storage. Furthermore, the higher dilution values of about 110 mls cannot be achieved using the above-described conventional approaches.
Various additional resins have been combined with the metal rosin resinates or added to the ink as dilution builders and also as binders in their own right. Highly phenol-modified rosins can be used in place of conventional rosins to achieve high dilution. However, these rosins are expensive and the resulting phenol-contaminated manufacturing waste must be treated or disposed as hazardous waste to avoid damage to the environment, which further increases the resinate cost. Cellulose derivatives are widely used in the industry to build ink dilution. These derivatives, especially ethyl cellulose and ethyl hydroxyethylcellulose, have very high molecular weights. However, they are very expensive and have poor compatibility with resinates.
It has recently been taught by Janusz, U.S. Pat. No. 4,690,712 (1987), that reaction products of a metal rosin resinate and an amino-polyamide are useful as vehicles for publication gravure printing inks. Dilution improvements are reported. In making such reaction products, the polyamide must have sufficient amino groups so as to be soluble in toluene and also to be able to react in the ratio of 1-5 equivalents of the amino-polyamide to 1-5 equivalents of the carboxyl groups of the metal resinate. This need for balancing the stoichiometry of amino and carboxyl groups poses reproducibility and even gelation problems, as well as requiring more of the relatively costly amine to be used relative to the less costly resinate acid. The solubility requirement severely limits the softening point and molecular weight of the amino-polyamide. Additionally, inks prepared with these polyamides are excessively thixotropic, which is undesirable for a fluid gravure ink.
The prior art also describes the use of high acid number, low molecular weight polycarboxylic polymers to improve resinate properties. For example, Schefbauer, in U.S. Pat. No. 4,244,866 (1981), teaches the use of alpha-olefin/maleic anhydride copolymers and partial esters thereof with limed rosin to prepare novel resinates. Schefbauer nowhere discloses achieving particularly high dilution. The polymers disclosed by Schefbauer are claimed to allow the preparation of resinates with very high lime levels. To achieve this end, the polymers must have low molecular weights and high acid numbers, typically over 130, and are used in relatively large amounts, typically 10 weight percent on a total solids basis. These polymers have poor toluene tolerance and, in fact, are used as solutions in 60/40 toluene/methyl ethyl ketone. This approach necessarily introduces an undesired solvent, a ketone, into the gravure ink in significant amounts.